Plastic wheels for vehicle tires and methods of injection molding those plastic wheels are known. Additionally, it is known to incorporate reinforcing fibers in the thermoplastic molding material to provide added strength to the molded wheels. However, several problems have been discovered in molding fiber-reinforced plastic wheels for vehicle tires. Typically, plastic wheels have lug apertures for receiving lug bolts or threaded lug studs and nuts to secure the wheel to a hub. The wheels also typically have a central hub aperture about which the lug apertures are circumferentially spaced. The radial spacing between each lug aperture and the hub aperture is such that weak areas are created in the web of the wheel. Thus, when the lug nuts are torqued or tightened in the lug apertures, the pressure tends to fracture the area inwardly radially adjacent to the lug apertures, rendering the entire plastic wheel unsuitable for use in mounting a tire to a vehicle hub.
The fracturing of the weakened regions inwardly radially adjacent to the lug apertures is believed to occur for essentially one of two reasons. First, the molding composition, which contains the reinforcing fibers, is injected into a wheel mold through circumferential gates in the injection molding apparatus, specifically a center molding form which defines the hub aperture. When the molding gates are radially aligned with the locations where the lug apertures are formed in the wheel web, at least a portion of the fibers in the molding composition flowing through the gates into the mold tend to orient radially in the wheel web region formed between the lug apertures and hub aperture. This radial orientation of fibers creates a line of weakness or fracture plane in the area inwardly radially adjacent to each of the lug apertures. Thus, when the lug nuts are tightened or torqued in the lug aperture, the plastic surrounding the lug aperture at the gate position may be more easily fractured.
Alternatively, when each molding gate is circumferentially aligned approximately halfway between the locations where two adjacent lug apertures are formed in the web, the molding composition flows through the gates and the fibers orient generally circumferentially. However, this results in "knit" lines forming at the points where the material injected from adjacent gates flows together. These "knit" lines represent undesirable lines of weakness or fracture planes in the plastic and, therefore, when these lines are formed in the web radially adjacent to the lug apertures, the plastic in these areas is subject to fracture upon tightening the lug nuts.
Therefore, a clear need exists for a method of molding plastic wheels which are substantially free of lines of weakness/fracture planes so as to reduce the susceptibility of the wheel to fracture in the areas radially adjacent to the lug apertures.